1. Field of the Invention
The present invention relates to accelerator apparatus provided in diaphragm carburetors supplying fuel to general purpose engines having limited displacement.
2. Description of Related Art
Many carburetors for supplying fuel to general purpose engines, particularly, general purpose engines having limited displacements, employ a diaphragm provided within a fixed fuel chamber regulating and holding the fuel supplied to an intake passage. The diaphragm maintains the fuel at a substantially constant pressure.
In such diaphragm carburetors, it is a known technology to supply an accelerated fuel to the intake passage in correspondence with an amount of intake air. The fuel acceleration is increased at a time when an opening degree of a throttle valve is increased for the purpose of increasing a rotational speed of an engine when a rapid acceleration is performed or when a load is applied. The diaphragm carburetor is provided with an accelerator apparatus using a piston-type or diaphragm-type accelerator pump, which operates mechanically with a throttle shaft having a throttle valve.
Further, in a known accelerator apparatus, as depicted in FIG. 2, an accelerated fuel passage draws the fuel to an accelerator pump activated by a cam 17 on a throttle shaft (not shown) having a throttle valve 2. Thus, an accelerated fuel flow is delivered directly to a main nozzle 9 open to a venturi tube 25 of an intake passage 3 from a fixed fuel chamber. Nevertheless, this accelerator apparatus is structured, such that the fuel is driven to venturi tube 25 through main nozzle 9.
Accordingly, when the engine is running in the full open throttle condition, the pulsation created by the engine affects to main nozzle 9. The pulsation may deliver an additional volume of fuel to the accelerator pump circuit through main nozzle 9. The pulsation effect may cause an unstable emission value, and in an undesirable case, the engine may not run properly because of this known “self pumping effect” when there is air or a fuel vapor in the additional volume. The pumping effect may discharge an excess amount of fuel into the engine, and the emission value may increase. In an undesirable case, the engine power declines as well due to the fuel rich condition.
When the engine starts with a dry carburetor condition, the accelerator pump circuit is filled with air, and the air may remain in the accelerator pump circuit until several throttle actions are executed.
Even after removal of the air from the accelerator pump circuit, there is a risk that the fuel vapor may be generated during the engine operation, due to hot ambient temperatures or a heat transfer from the engine, or both. There also is a risk that the self pumping effect may be generated by the engine vibration. Further, unstable emissions (e.g., unstable air-to-fuel ratio) are a problem for engine manufacturers due to an emission regulations.